Date of Award

12-2011

Level of Access

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical and Computer Engineering

Advisor

Mauricio Pereira da Cunha

Second Committee Member

M. Clayton Wheeler

Third Committee Member

Bruce Segee

Abstract

Hydrogen fluoride (HF) is a hazardous compound that is used in a variety of industrial processes and is also a decomposition product of many environmentally harmful fluorinated volatile organic compounds. Surface acoustic wave (SAW) resonators on quartz (SiO2) substrates are suited for HF sensing because the analyte reacts directly with the device substrate. Platinum (Pt) electrodes are utilized for their high corrosion resistance to HF.

The dominant HF sensing mechanism of Pt-electrode SAW resonators on quartz is identified in this work as a response to the mass loading effects of a liquid layer generated by the HF/SiO2 reaction, rather than material removal from etching of the substrate. The use of Pt electrodes is shown to reduce the penetration depth of a shear horizontal SAW (SH-SAW), along quartz ST-900, beyond that of a generalized SAW (GSAW), along quartz ST-X, thereby making the pure SH-SAW more sensitive to surface perturbations and thus, the generation of the liquid layer.

Short-circuit grating resonators utilizing Pt electrodes on quartz ST-X are shown to leak acoustic energy from the grating region to the bus-bars, thus degrading the resonator response. To guide the GSAW in the acoustically active grating region of a Pt-electrode resonator on quartz ST-X, open-circuit gratings are proposed and implemented. This improvement in the resonator design increased the quality factor and decreased the loss of measured responses when compared to short-circuit grating resonators.

Spurious transverse waveguide modes are identified in the responses of open-circuit grating resonators on quartz and other substrates by deriving a general dispersion relation that accounts for asymmetry in the slowness curve around the propagation direction. The dispersion relation reduces, as a special case, to that of a symmetric slowness curve. In addition, the identified spurious transverse waveguide modes are mitigated by scaling both the transducer's grating aperture and the electrode overlap width.

The decomposition of the hydrofluorocarbon, R-134a, into the products HF and CO2, is explored at elevated temperatures using a supported Pt catalyst in a fixed-bed reactor. The Pt-electrode SAW resonator developed in this work successfully responds to the HF generated by the decomposition of R-134a.

Share